This does not happen all the time, but just did now - I got an email. Its summary [the from/subject and 3 lines of text] show up just fine in the middle column of Mail but when I click on it on the R side column nothing is in the body text. And if I open it individually nothing is there and if I try to Forward it only the headers show up as being forwarded.
But the middle summary information is there and is all fine. But it's obviously not the whole message.